The need for continuous duty, high power rotating anode x-ray tubes exists in medical radiography, i.e., fluoroscopy and computerized tomography (CT), and in industrial applications such as x-ray diffraction topography and non-destructive testing.
A number of schemes have been proposed in the past to achieve continuous power output at high peak power with a rotating anode x-ray tube. These include direct liquid cooling of the anode, liquid to vapor phase cooling of the anode, as well as other techniques.
Examples of rotating anode x-ray tubes using other than liquid cooling are described in U.S. Pat. No. 4,165,472 issued Aug. 21, 1979 to Wittry (liquid to vapor phase cooling in a sealed anode chamber), U.S. Pat. Nos. 3,959,865 issued to Koncesznski on May 25, 1976, 3,719,847 issued to Webster on Mar. 6, 1973, 4,146,815 issued to Childenc on Mar. 6, 1973 (melting or vaporization of inserts in solid anode), and 3,736,175 issued to Blomgen May 22, 1973 (heat pipe to external heat sink).
Such rotating anode x-ray tubes have proven to be less efficient than direct liquid cooled tubes, and sometimes have a tendency to burst or explode when overheated, rendering such tubes unsafe.
Liquid cooled rotating anode x-ray tubes are, in general, well known. In such x-ray tubes, a hollow anode is disposed so that a rotating portion thereof is irradiated by an energy beam (e.g. electron beam). The irradiated portion of the anode is generally referred to as the electron beam track. Substantially all of the heat generated by irradiation by the energy beam is transmitted to a heat exchange surface, typically the interior wall of the hollow anode underlying the electron beam track and adjacent areas. In other words, the heat exchange surface is generally an area of the interior surface of the anode larger than the electron beam track, centered on and underlying the electron beam track. A flow of liquid coolant is passed into contact with the heat exchange surface to remove the heat therefrom, and thus cool the anode.
The basic cooling mechanism in liquid cooled anodes for use in x-ray tubes is nucleate boiling (or other vapor or gas mechanism). In nucleate boiling, bubbles of vaporized fluid are generated on the anode heat exchange surface. The vapor bubbles break away and are replaced by fresh bubbles, much like a pot of boiling water, thus providing efficient cooling by the removal of heat from the exchange surface to vaporize the liquids.
However, under certain circumstances, the power handling capacity of the system is limited by transformation of the nucleate boiling mechanism into what is known as a destructive film boiling phenomenon (or other vapor or gas blanket). The heated surface becomes surrounded by an insulating vapor blanket, thus causing significantly reduced heat transfer. The primary heat removal mechanism therefore becomes radiation and convection through the vapor.
The heat flux at the transition from nucleate to film boiling is called the critical heat flux. Should this value be exceeded in electrically heated structures such as a liquid cooled x-ray tube anode, the insulating film blanket would cause a rapid rise in temperature, typically resulting in burn out (i.e., melt down) of the structure. In general, burn out occurs very quickly, and the protective means required are extremely elaborate and expensive. Thus adequate protection has not heretofore been practical.
Formation of the boiling film occurs when expanding bubbles are generated faster than they can be carried away. The expanding nucleate bubbles interact and combine ultimately to form an insulating blanket of vapor. Thus, the transition is made from nucleate boiling to film boiling. It is therefore the bubble interaction which controls the heat transfer process.
To provide for efficient heat removal from the liquid cooled inner surface of the anode, i.e., at the anode heat exchange surface, a high relative velocity between the anode heat exchange surface and the liquid approximately 50 feet per second or greater, is required. In the past, high pressure pumps have been used to achieve the desired high liquid flow rates. However, use of high pressure pumps result in shortened rotational fluid seal life and attendant anode design limitations as will be hereinafter more fully discussed.
Examples of prior art liquid cooled rotating anode x-ray tubes are described in: Philips Technical Review, Vol. 19, 1957/58, No. 11, pp. 314-317, U.S. Pat. Nos. 4,130,773 issued to Kussel et al on Dec. 19, 1978, and 4,238,706 issued to Yashibara et al on Dec. 9, 1980 and A. Taylor, "High-Intensity Rotating Anode X-Ray Tubes", from Mallet et al, Advances in X-Ray Analysis, Vol. 9, Plenum Press N.Y. pp. 194-201. (describing the "Taylor device"). The rotating anode of the Philips device constitutes a hollow cylinder with three radially running tubes through which water flows to a cavity located along the inner surface of the peripheral wall or anode strip of the hollow body. In this device, the water flows back into the hollow drive shaft through three other tubes running radially in the rotary anode. However, various disadvantages have been attributed to the Philips device. For example, it is reported that only relatively low speeds of rotation can be obtained with the Philips rotary device because the maximum thickness of the peripheral wall provided as the anode target member allowable for proper cooling is not sufficient to withstand the pressures in the cooling medium that arise due to centrifugal force at higher speeds of revolution. Only relatively small surface density of illumination (brightness) can be obtained with this known rotary anode, since the intensity of illumination, i.e., radiation per unit of surface, generated by a device depends upon the rate of anode revolution.
The Kussel et al patent describes a liquid cooled rotating anode which purports to resolve the shortcomings of the Philips device. The portion of the rotary anode cylindrical peripheral wall, whereupon the electron beam strikes, is cooled with water supplied and removed, respectively, through coaxial ducts distributed by radial ducts in one end face of the rotary to a ring duct and gathered from a ring duct at the other end face through another set of radial ducts leading back to the shaft. Between the two ring ducts, the cooling medium flows through helical cooling ducts running parallel to each other and at an angle of about 15.degree. to the edge boundaries of the cylindrical operating surface. These ducts are formed on the outside by the anode peripheral wall material itself and on the inside by a stainless steel insert with grooves machined thereon.
However, in order to obtain efficient heat transfer, relatively high coolant velocities are required with Kussel et al device. To achieve high coolant velocities, high pump pressures are needed. Unfortunately, the seals necessary to join stationary to rotating fluid conduits generally have short lives when subjected to such high coolant pressures and high speed anode rotation.
A more basic limitation of the Kussel et al device arises from the use of the grooved stainless steel insert to form the coolant ducts. The outermost rims of the groove walls are brazed to the anode peripheral wall. As described, the cooling ducts traverse one face of the anode to the other at a pitch angle of 15.degree.. The brazed duct wall peripheries thus also transverse one face of the anode to the other at the prescribed 15.degree. angle. Therefore, the electron beam alternately travels over coolant duct and duct wall as the anode rotates. When the electron beam is above the coolant, heat transfer is efficient, whereas when the beam is above the duct wall, the anode simulates more closely a solid metal structure, i.e., a conventional solid rotating anode. This creates a hot spot and severely limits the power handling capability because of the long heat path to the coolant. The braze alloy, used to braze the anode to the insert melts well below the metals used in the anode, and this further limits the power densities that can be handled. The duct walls, brazed to the periphery of the anode, which provide the necessary strength to the anode shell to prevent its distortion due to centrifugal force of the coolant, become a liability in that they become a limiting factor in power handling capability.
In the Taylor device, described in Advances in X-Ray Analysis, supra, liquid coolant flows in a direction transverse to anode rotation and interacts with the the anode in a manner known as "linear coolant flow". However, although there is a high relative velocity between the anode and coolant, the interaction is relatively inefficient and is reported by Taylor to provide only relatively low power (71/2 kw). This stands in sharp contrast to the 100 kw attributed to the Kussel design. However, the Taylor design is not subject to performance-limiting centrifugal forces as the Philips device is, and permits the use of low pressure pumps and components.
Further, description of prior art liquid cooled rotating anode x-ray tubes is found in the following articles: G. Fournier, J. Mathieu: J. Phys. 8, 177 (1973), R. E. Clay: Proc. Phys. Soc. (London) 46,703 (1934), R. E. Clay, A. Miller: J. Instru. Elect. Engs, 84,261 (1939), W. T. Astbury, R. D. Preston: Nature 24,460 (1934), Z. Nishiyama: J. Japan Met. A. Soc. 15,42 (1940), V. Linnitzki, V. Gorski: Sov. Phys-Tech. Phys. 3,220 (1936), R. R. Wilson: Rev. Sci. Instr. 12,91 (1941), S. Miyake, S. Hoshino: X-sen 8,45 (1954), (Japanese) Y. Yoneda, K. Kohra, T. Futagami, M. Koga: Kyushu Univ. Engs. Dept. Rep. 27,87 (1954), S. Kiyono, M. Kanayuama, T. Konno, N. Nagashita: Technol. Rep., Tohoku Univ. XXVII, 103 (1936), A. Taylor: J. Sci. Instr. 26,225 (1949), Rev. Sci. Instr., 27,757 (1956), D. A. Davies: Rev. Sci. Instr. 30,488 (1959), P. Gay, P. B. Hirsh, J. S. Thorp, J. N. Keller: Proc. Phys. Soc. (London) B64,374 (1951), A. Muller: Proc. Roy. Soc. 117,31 (1927), W. T. Astbury: Brit. J. Rad. 22,360 (1949), E. A. Owen: J. Sci. Instr. 30,393 (1953), K. J. Queisser: X-ray Optics, Applications to Solids Verlag-Springer, NY (1977), Chap. 2, Longley: Rev. Sci. Instr. 46,1 (1975), Mayden: Conference on Microlithography, Paris, June 21-24, 1977, pp. 196-99, MacArthur: Electronics Eng. 17,1 (1944-5), A. E. DeBarr, Brit. J. Appli. Phys. 1,305 (1950).
In general, it is known that stable flow patterns in the coolant of a liquid cooled system inhibit the removal of nucleate bubbles and result in a substantial reduction of the cooling efficiency of the system. For a detailed discussion of flow patterns see Greenspan, The Theory of Rotating Fluids, Cambridge Press, 1969. It has now been discovered that the coolant flow fields generated with the anode of conventional liquid cooled rotating anode devices have such inherently stable rotational motion which results in a substantial reduction of the power rating of the tube.
Such stable flow patterns are established by the fact that the liquid adjacent the rotating anode surface has a high velocity induced in it, whereas the liquid adjacent the stationary inner cylinder i.e. the septum has a low velocity. In turbulent flow, the velocity gradients from both the stationary surface (septum) and rotating surface (anode) are high. Since the liquid at the largest radius (at the anode surface) has the highest velocity, it therefore has the highest centrifugal force associated with it. This forces the liquid outward against the anode surface. Thus, the water cannot be inwardly displaced and liquid circulating patterns are inhibited. The large velocity gradients, i.e. centrifugal force gradient at the anode surface further aggravates the problem. Thus, the liquid flow pattern is literally in the "grip" of the rotating anode and a stable flow pattern is established that must be broken up or prevented to enable liquid to circulate at the anode surface to facilitate removal of nucleate bubbles which will increase heat transfer. The viscous or laminar sublayer--a thin layer of laminar flow adjacent to the wall of the conduit and always present in turbulent flow--provides a mechanism to further cause the nucleate bubbles to adhere more readily to the anode surface.
The rate of nucleate bubble removal may be increased by breaking up this viscous or laminar sublayer. As described in co-pending U.S. Application Ser. No. 250,275, filed on Apr. 2, 1981 by Arthur Iversen, such viscous sublayer can be broken up by roughening the anode coolant surface. When the height of the roughening projections ranges from 0.3 times the thickness of the viscous sublayer to the sum of the thickness of the viscous sublayer and a transition zone adjacent the viscous sublayer, the sublayer is broken up. Breaking up the viscous sublayer enables the turbulent fluid to reach the base of the nucleate bubble, where it is attached to the anode, thereby providing the energy needed to break it loose. The geometry of nucleate bubbles is a function of the surface roughness geometry; small fissures tend to generate small nucleate bubbles, whereas large fissures tend to generate larger ones. Therefore, nucleate bubble size and generation can be optimized by providing a surface of calculated and preferably uniform roughness and geometry. A regular roughness geometry can be obtained by suitable conventional techniques such as, for example, chemically by means of chemical milling; electronically, by the use of lasers or electron beams; or mechanically, by broaching, hobbing, machining, milling, stamping, engraving, etc.
Another method of obtaining a surface with crevices for forming nucleate bubbles is the use of a thin porous metal layer adherent to the anode at the anode heat exchange surface. This porous metal layer may be considered to provide a contoured surface as defined above. Relatively uniform pore size can be obtained by fabricating the porous structure from metal powders with a narrow range of particle sizes. Methods, such as described in U.S. Pat. No. 3,433,632, issued to Ebbertotal on Mar. 18, 1969, are well suited to providing the desired porous metal structure.